/Engine/Source/Runtime/Engine/Classes/GameFramework/OnlineSession.h
Called when a user accepts an invite
virtual void OnSessionUserInviteAccepted ( const bool bWasSuccess, const int32 ControllerId, FUniqueNetIdPtr UserId, const [FOnlineSessionSearchResult](API\Plugins\OnlineSubsystem\FOnlineSessionSearchResult) & InviteResult )